A Dark 'S' Erupted!

Science Nugget: February 2, 2001

What does a dark 'S' mean?

Recently, bright S's have been popular among solar physicists,
especially those who study flares and CMEs.
These S's are called 'sigmoids,' and they are usually observed as
bright structures in soft X-rays.
They are known to be a signature of highly sheared magnetic field
in an active region,
which thus has a high potential for energetic flares.
You can find many
images and
articles on the Web.

However, the 'S' here is not an X-ray sigmoid but one falling into a
rather traditional category: a dark filament.
The one observed during last week(4th, 2001) showed a very gorgeous
'S' shape in SoHO/EIT images in the extreme ultraviolet.
It can, of course, be seen in routine
H-alpha image as shown below (unfortunately, it does not appear so
perfectly S-shaped as the EIT version does).
It was clearly located on a magnetic inversion line, which bends like
an 'S' around the region in question, as also shown a image below.
Observed by Yohkoh/SXT, however, it is not distinct at all.

What is interesting is that this dark 'S' erupted near the west limb and
caused a flare just like an X-ray sigmoid may do.

What happened?

During the week 4, 2001, the activity on the sun was not so high,
although a few M-class flares were reported.
Instead, several long filaments crawled quietly on the disk, and
one of them strongly drew one's attention because of its beautiful S-shape.
However, it was too faint in soft X-rays to be chosen as Yohkoh's
observation target, which is usually set to the brightest region on the
disk.
It seemed to pass across the disk and go away with its stylish shape remaining.
However, it was an old fox!
It did erupt just before hiding away behind the limb,
and Yohkoh caught its tail!

As you may see in the above movies, the evolution of the filament is identified
in the HeI (low-temperature) images of SoHO,
while the formation of arcades accompanying the
filament eruption was mainly observed by Yohkoh/SXT because these
are high-temperature structures. It appears that the filament erupted at the
time before 1:18 UT, but we cannot tell exactly due to the
low cadence of the EIT images.
The SXT arcades started to form around 1:30 UT and became brightest
around 3:00 UT.

What have we learned from this event?

Judging from the GOES flux variations (flares), it was a mediocre and boring
week, but from a CME point of view it was highly active and exciting.
Actually, this event resulted in one of the major CMEs within those few days.
Have a look at the
LASCO MPEG movie. The CME
associated with the eruption of the dark 'S' is the one early on
January 26th.
This CME was accompanied only by a small peak in
soft X-ray flux (C1.6, around 3:00 UT, 26-Jan).
Even this was just a coincidence; the SXT images show that an entirely
different region flared.
The CME-related arcade appeared clearly in the X-ray images,
but type of CME counterpart could hardly be termed outstanding,
because of the faintness of the arcade.
This event shows an example of a type of CME, for which the soft X-ray
morphology, not just the X-ray brightness, should be used in a
forecast.

This may be further evidence for the recent exciting work of L. Van Driel
and collaborators, who are studying the "last best" region AR 9398 from
1996.
They find that flare productivity and CME productivity seem to be somewhat
uncoupled, and they offer the suggestion that flares may represent
relaxations following energy buildup, whereas CMEs may represent
relaxations following magnetic helicity buildup.